Grassfed butter is better for your teeth and bones because it contains higher levels of vitamin K2. K2 is one of key fat soluble vitamins (along with A and D) responsible for transporting the minerals into our bones and teeth. See my "WAPF fed" link in my sig. below why this is important for your child's teeth, bones and facial structure. You can drink lowfat milk til the cows come home and you won't absorb the calcium well without the fat and the fat soluble vitamins... traditional diets reveal the fallacy of our modern obsession with lowfat foods.
Organic Valley also has a pastured organic butter packaged in green foil, we use this when I don't get our raw Amish pastured butter. Also Purity Farms organic ghee, Smjor butter from Iceland, Anchor butter from NZ and Jana Valley butter are grassfed.
Also egg yolks from pastured hens (where they eat a lot of bugs) have high K2. Our yolks are a crazy orange color which signifies high K2 just like very yellow butter is the highest as well. This time of year even our raw cream and milk is yellow!
Shellfish is also high in K2. Fish eggs (super high). Organ meats.
You are can also buy concentrated high vitamin butter oil from Green Pastures if these foods are not available to you.